Default download attribute fix

Fix for that makes the extension works with the default.
https://github.com/chrisrobins/YouTubeHTML5/commit/3644cd5960431bb220595c80df6626f98bfcd15a#commitcomment-4977023
Made this to make @chrisrobins saves ... seconds, and also get famliar to GitHub.
If someone could explain what I've done wrong to get the Merge pull request appears in the commit list...